In electrophotographic image formation, an electrostatic latent image is formed on an image bearing member comprising a photoconductive material, and the electrostatic latent image is developed into a toner image with a charged toner. The toner image is then transferred onto and fixed on a recording medium. In the field of electrophotography, full-color copiers and printers have been brought to the mainstream in place of monochrome copiers and printers recently.
In a typical full-color image formation, toner layers of yellow, magenta, cyan, and optional black are superimposed on one another to reproduce various colors, and the resulting composite toner image is finally fixed on a recording medium. To obtain a vivid full-color image which is precisely reproducing colors, the surface of the composite toner image fixed on the recording medium is preferably as smooth as possible so as to reduce light scattering. Thus, a typical full-color image has a middle to high image gloss level of 10 to 50%.
Typically, a toner image is fixed on a recording medium by pressing a heated roller or belt against the toner image on the recording medium. Such a fixing method may be called as contact heating fixing method. The contact heating fixing method provides high thermal efficiency and high-speed fixing, thus producing images with high gloss and transparency. However, because the heated roller or belt is pressed against the melted toner image and then separated therefrom, the method causes an undesirable phenomenon in which a part of the toner image is adhered to a surface of the roller or belt and retransferred onto another image. This phenomenon is hereinafter called as hot offset.
To prevent the occurrence of hot offset, there has been a proposal to form the surface of the roller or belt with a material having high separability (repellency) such as a silicone rubber or a fluorine-containing resin and further apply an offset preventing oil such as a silicone oil to the surface. This proposal has been widely employed. The offset preventing oil effectively prevents the occurrence of hot offset but requires an apparatus that applies the oil to the roller or belt. Therefore, the fixing device undesirably becomes larger and larger.
In view of this, toners for forming monochrome images have been developed to include a release agent and to express a large viscoelasticity when melted. Such toners can be used for fixing systems in which no oil or a slight amount of oil is applied to the fixing roller or belt (hereinafter “oilless fixing systems”).
Recently, compact and simple oilless fixing systems are spreading in the field of full-color image formation technology as well as in the field of monochrome image formation technology. However, it is likely that hot offset more frequently occurs in full-color image formation because toners are designed to express a lower viscoelasticity when melted so that the fixed toner image has a smooth surface. Therefore, it is difficult to employ oilless fixing systems in full-color image formation. Disadvantageously, toner particles including a release agent are transferred onto a recording medium at a lower transfer rate due to their high adhesive property. Further, such toner particles may make thin films thereof on carrier particles (hereinafter “filming”), resulting in deterioration of chargeability and durability.
On the other hand, there have been various attempts to provide a low-surface-energy covering layer, comprised of a fluorine-based resin, a silicone resin, or the like, on a core material of carrier, for the purpose of preventing the occurrence of filming, forming a uniform carrier surface, preventing oxidation of the carrier surface, preventing deterioration of humidity resistance, extending the lifespan of two-component developer, preventing adherence of carrier to photoreceptor, protecting photoreceptor from scratch or abrasion, and controlling charge polarity and quantity.
On the other hand, disadvantageously, recent toners are more adhesive to carrier particles because they have a smaller particle diameter and are subjected to a higher-speed printing. Such toners further including a release agent are much more adhesive to carrier particles. In this case, toner charge is so reduced that toner scattering and background fouling easily occur.
When toner particles adhere to carrier particles or covering layers of the carrier particles are abraded or peeled off, electric resistance and toner supplying power of the carrier particles are changed. As a result, the resulting image density is changed, especially in highlight portions, and the resulting image is contaminated with fillers released from the covering layer, especially in yellow images.
In accordance with reduction in particle size of toner, carrier has been also reduced in particle size, which undesirably accelerates the occurrence of carrier deposition. To improve image quality, electric resistance and magnetization of carrier should be properly controlled, however, it is difficult to change them while maintaining compatibility with developing device in which the carrier is to be used.
Carrier deposition is a phenomenon in which carrier particles are adhered to an electrostatic latent image bearing member (e.g., photoreceptor) and then deposited on an image. Carrier deposition generally occurs in a whole area, a background area, or an image area. Carrier deposition occurred in a whole area is due to low magnetic force of carrier particles. Carrier deposition occurred in a background area is due to high resistance and large particle diameter of carrier particles. Such carrier particles are reversely charged to toner particles and thus reversely deposited. Carrier deposition occurred in an image area is due to low resistance of carrier particles. Such carrier particles are deposited together with toner particles because toner charge or developing charge is injected into the carrier particles. Lower-resistance carriers generally provide higher image quality, however, more cause carrier deposition due to charge injection. Thus, the occurrence of carrier deposition should be prevented by controlling the shape of carrier particles.
For example, carrier particles substantially having a spherical shape have been proposed to prevent carrier deposition.
However, there is a possibility that the covering layer causes blocking or deteriorates durability. Moreover, carrier resistance cannot be well controlled only by carrier shape.